Can One Use the Static Charge Collected by the Antenna to Charge a Battery?

Sunday, 01 April 2012 00:00
oh7ug
Techie

In a comment to my article on Antenna Static Charge Bleeder, reader Spicer raised a question: could one use the static charge collected by the antenna to for example charging a battery. I had bad experiences of having static in the antenna during snowfall, see also Static Discharge from Antenna Can Do Damage to Your Gear.An antenna will collect charge from charged particles hitting it, snowflakes and raindrops. It also acts as a capacitor to store the charge. A bleeder resistor may be used to to drain the charge before it builds up and makes any damage.

If there is nothing in place to drain the collected charge from the antenna, it may be manifested with high voltage and arcing at the equipment end. This is because the capacitance of the antenna is small. If it were larger, the voltage potential against ground would be lower.

Arcing and associated high voltage spikes caused problems at my linear amplifier. In the amplifier the SWR –measuring circuitry consists of a toroid coil (transformer), picking its primary voltage from the antenna lead, to a pair of Germanium diodes and other components. The spikes were too much for the diodes, which I had to replace - and figure out a system to prevent this of happening again - I came out with simple solution: a bleeder resistor.

Reader's Question: Using the charge collected by the antenna?

In theory one could use this collected charge and its stored energy to charge, say, a battery. However there are some obstacles for practical use.

- the charge collected is very, very tiny; It would take a very long time to collect enough to charge a battery

- the voltages associated are very high; it would be difficult if not possible at all to find a practical circuitry to lower them to the battery level and simultaneously maintain the small stored energy

Back to the school bench and do some analysis

Charged particles are either negative or positive, so the system is DC, making the analysis easier. Let’s think of charging a small rechargeable AA cell. Typical rechargeable AA battery cells are rated with a charge between 800 to 3000 mAh. Let’s assume we want to charge a 1.2 Volt and 2400mAh battery cell from empty to full. This means we need 2.4Ah or 2.4x3600As or Coulombs of charge moved into it (1 Ampere = 1 Coulomb/second).

A closer look

If the antenna capacitance is say 200pF and the potential (voltage) between antenna and the ground when charged, just before arcing, was 6kV, and arcing happened twice per second, we can calculate the amount of charge and energy stored and released in the system. Let’s also assume the charge collected is more or less uniformly distributed across the whole antenna. Also we assume a steady flow of charged particles hitting the antenna from snowing/rainfall.

Now the charge collected by the antenna capacitor raises the voltage to say 6kV before arcing. Electromagnetic theory tells: Q = C x U where Q = charge stored in the capacitor, U = potential across the capacitor, C = capacitance. This implies U = Q / C, or the smaller the capacitance, the larger the voltage for any given amount of charge.

Collected charge and current to the antenna

For the antenna in case: QAnt = CAnt x UAnt = 200x10-12 x 6000 Coulombs = 1.2 x 10-6 C = 1.2μC (microCoulombs). This charge is collected twice per second, so the charge collected from snowfall/rain in unit time of one second will be 2.4μC.

Assuming the flow of charge to the antenna is steady and 2.4μC/s, then the current IAnt to the antenna wire, which is defined as amount of charge moved per unit time or I = Q / T, would be 2.4μA. This would be across the whole exposed antenna. Note that this implies Q = I x T, so the antenna cumulative charge in a second can also be expressed as 2.4μAs.

Direct battery charging with the charge collected?

If we could somehow move this charge flow (=current) hitting the antenna without losses directly to a battery cell, ie. charging it from “empty” to full with this steady 2.4μA current, it would take 2400 x 10-3Ah / (2.4 x 10-6A) = 106 h = 1 million hours, 114 years – or a couple of lifetimes. Really a trickle charging. Obviously a direct connection moving the charge to the battery would not be efficient.

Usable energy of the stored charge in the antenna capacitor

However, remember that we have a capacitor in place – and high voltage. Theory tells that the energy which can be extracted from a charged capacitor is E = 1/2 x C x U2.The energy, in form of stored charge and voltage potential in the antenna capacitor, just before arcing, would be EAnt=1/2 x CAnt x UAnt2 = 0.5 x 200x10-12 x 60002 Joules = 3.6 x 10-3 Joules = 3.6 milliJoules = 3.6 milliWattseconds. We would have this energy available 2 times per second, or total of 7.2 mWs.

The usable energy in a 1.2 Volt 2400mAh AA cell is roughly 1.2V x 2400 mAh = 2.88 VAh = 2.88 x 3600 Ws = 10368 Ws (or Joules). This energy is the same as the potential energy of 1 kg mass at about 1km height, ie. in ideal situation you could lift 1 kg mass up 1km in the air with an AA cell. Pretty impressive!

Antenna energy vs battery energy

Now when taking the ratio of the above antenna capacitor’s charged energy to AA battery’s energy, one finds that the AA cell has roughly 1.44 million times more energy stored, than is collected by the antenna capacitor in a second.

This means that the antenna should collect, and after reaching 6kV, store forward the charge twice a second, for over 1.44 million seconds, for the stored energy to reach the level inside of an AA-cell. This is nearly 17 days. A long time still. I would not like to live in a place where there were constant raining/snowing for 17 days or more in a row .

The apparent difference between 1 million hours and 1.44 million seconds comes from the assumption that the charge in the antenna would be at 6kV potential instead of 1.2 Volts and we could convert the energy it contained into energy in the battery.

Any practical circuitry?

The second obstacle would be how to realize a practical charging circuitry – from 6 kV down to 1.2 Volts, without huge losses.

For example, one might just think of adding a larger capacitor C2 to the bottom of the antenna, where the collected charge would be moved. This capacitor would be switched in as soon as the voltage reaches the arcing level. Following the U = Q / C law, we could lower the voltage to closer to the battery charging voltage. This capacitor would get the charge twice per second.

Let's assume we want to lower the charged antenna system potential to 2V, to be able to charge a 1.2 Volt battery cell. Then a charge amount of 2.4μC will be moved from the antenna capacitor to a second larger capacitor C2 = QAnt / U2 = 2.4 x 10-6 / 2 = 1.2 x 10-6 Farads = 1.2μF

Which is actually quite normal capacitor for 2 volts (but not for 6 kV). The 200pF antenna will be negligible when in parallel with this. However the energy usable from C2 will be much less as it follows E = 1/2 x C x U2 law.

E2 = 1/2 x C2 x U22 = 0.5 x 1.2x10-6 x 22 = 2.4x10-6 Joules = 2.4 microJoules or 1/3000th of the energy usable from the antenna capacitor. Accordingly charging the battery would take at least 3000 times longer and we come back close to a million hours.

Conclusions

Better stick to using a bleeder resistor to drain the charge than trying to use it for something else. The "else" might be damaging you connected electronics. There can be several millijoules of energy available in the static charge. Typical modern microelectronics can easily be damaged with this - as I have experienced. Fortunately the static rain is quite rare here. Finally, keep in mind, lightning is a different beast with huge amount of energy released, the bleeder resistor is no protection against it.

Of course this is nothing new. Utilizing the static - or friction electricity has been studied for over 200 years - without many practical results. If there were efficient means of extracting energy from static, they would have been invented a hundred years ago. But for me this was good and delighting exercise to refresh some math/physics learned some 30+ years ago.

Last Updated on Wednesday, 18 April 2012 18:50

Reverse Beacon Network as a Tool to Check Propagation and Who Can Hear Your Signal

Tuesday, 25 January 2011 15:15
oh7ug
Antennas

Alex, VE3NEA has made several nice sw tools for ham use. One of them is CW Skimmer. As I do not have SDR and have not yet connected my radio to the PC, I have not used it myself, but there is a growing ham community group using it also for other ham's benefit - mine, too.

Felipe, PY1NB has set up a web site (www.reversebeacon.net) where you can see from a real time World map who are CQing on the bands (on CW). And how well the signals are heard. The site aggregates CW Skimmer information from hams all over the world. You can restrict the view to certain bands and also search for specific callsigns. I took a screenshot while CQing on 40m using the new vertical setup. My station is the red spot in the top middle (South Finland) heard in Europe, Australia, Japan and Iceland. The listing shows whose CW Skimmer was listening and what was the signal to noise ratio.

Last Updated on Tuesday, 25 January 2011 16:29

New "Shortened" 26 m Siderpole Fiberglass Vertical

Monday, 20 September 2010 00:00
oh7ug
General

The new 26 m Spiderpole fiberglass antenna arrived late June. Due to holiday trips and other Summer activities I got it up on 7th August. I wanted to have a support for vertical and possible wire antennas. It should be easy to handle and set up, but be something more rigid than the earlier 18 m Spiderpole which broke during a storm earlier this Summer. A good, though more costly, solution was to deploy the new 26 m Spiderpole. I have taken off three topmost sections (whip), leaving total of 12 sections of the telescoping pole. Total height is somewhat above 20 m. Tip diameter is 30 mm and bottom about 110 mm.

There are 30 radials (variying from 4 to 20 m in length) dug 2-3 cm into the ground, using bare 2,5 sq mm household wire (the insulation stripped off). Currently I have about 600 m of wire in the radial system. The house and nearby lot border restrict layout of the radials. They cover only about 2/3 of full circle in a bent hourglass shape.

2 mm OD enamelled copper wire runs inside the pole. I have pulleys at top and topmost guying point for easy installation of wire antennas. The top cover and pulley are assembled with a 30 mm rubber boat plug, some steel strap with heat shrink rubber and a clamp - all stainless steel.

The pole is guyed with three guy lines at three levels. The guy lines are 3 mm OD Dyneema racing boat rope breaking strength over 450 kg.

Three guying belts came with the 26 m Spiderpole for easy guy line attaching. The belts are a good place to attach pulleys for wire antennas, too.

Three guy wires are attached to belts at each level, 120 degrees apart. Try to get the topmost angle at least 30 degrees to ensure rigidity and lower the stress to pole. At 30 degrees the horizontal wind pressure gives 1,7 times more stress towards ground along the vertical pole and 2 times more along the guy line. This comes from simple trigonometry and Newtons laws. To overcome the stress along the pole I added a second clamp above each section below the bottom guying point.

The pole got it's wind test one day after setting it up, when Helsinki area was hit with a heavy thunderstorm, wind speeds reaching 30 m/s. The pole survived. It is remarkably more rigid and stable than the previous 18 m pole.

Last Updated on Saturday, 22 January 2011 20:03

"Shortened" 26 m Spiderpole up

Thursday, 09 September 2010 12:22
oh7ug
Antennas

My 18m high Spiderpole broke earlier this Summer in a thunderstorm gust. I have now replaced it with a new 26m Spiderpole with top three sections taken off giving total height a bit above 20 meters. I renewed guying using the guying belts which came with the pole. The old 18m pole was too flexible for "near" permanet use in my environment. Shortened 26m pole is remarkably stronger and more stable even in high winds.

The new pole got it's first challenge just one day after I got it up. On Sunday the 8th August, by the end of a very nice sunny day, we got a huge thunderstorm in Helsinki area with winds reaching 30 m/s. I have to admit getting a bit nervous when I saw the sky turning dark and the almost black storm cloud front approahing like an enormous tsunami wave in the air - and coming over us extremely fast. With the front came fast streaming gusts. But the antenna survived. The only thing needed was checking the guy lines a couple of times during the storm.

The storm toppled lots of trees and caused blackouts for more than 70000 households in Southern Finland.This is a link taken at a beach in Helsinki when the storm came: http://www.youtube.com/watch?v=KzGk_NXlobQ

Last Updated on Monday, 20 September 2010 17:48

Broken 18m Spiderbeam Pole

Monday, 14 June 2010 07:46
oh7ug
Antennas

The 18 m Spiderbeam pole mast is made from telescoping sections which are secured with clamps. There are 12 sections each 150 cm when extended. I'm using it as 80 m vertical antenna. The pole is fastened up 2 meters to the house wall, tip being at 20 m level. The guy attachments were above 4th, mid 6th and above 9th sections, or 8m, 11,75m and 15,5m levels from ground. The guy wires are 3 mm Dyneema (similar to Spectra) yacht racing wire, being in place for nearly 4 years, so pre-stretched.

This weekend we had extremely high winds with gusts reaching well above 20m/s speeds. I saw the pole wobbling between guying levels in the gusts.

On one of the gusts the pole suddenly bent about 90 degrees, most of the pole above the first guy level being about horizontal (I saw it happen). I had seen this before with two sets of guys (above 5th and 9th section), and had added one more set of guys to stabilise the pole.

The pole broke at three points. At lowest guy point above 4th, 1/3rd way up 6th and just above sixth section. The first one to go may have been the 1/3rd way up 6th section and the two joints breaking caused by the collapse and the pull shock from guy wires.

It looks like the 7th section had slid down about 5-7 cm due to wind stress and perhaps the clamp not being tight enough. Both this and the wobbling may have caused guy wires to slack a bit too much and allowed the pole to bend in the wind gust. The guy wires were intact and none of them broke.

I will replace the pole, but I'm considering to move to use the new 26m pole instead and take the top 4 sections off. Alternatively I could take the bottom section and three top ones off. I am worried of the 18 m pole being too flexible and wobbly in gusts.

Using the the 26 m pole for 80m full size means to reduce it to 20m tall, but I could leave the bottom section on ground level instead of lifting up 2 meters (as with the 18m pole). We'll see. But now I will be QRT for a few weeks :o(